This application claims benefits of Japanese Application No. 2001-351624 filed in Japan on Nov. 16, 2001, the contents of which are incorporated by this reference.
The present invention relates generally to a zoom lens and an electronic imaging system, and more particularly to a zoom lens, the depth dimension of which is diminished by providing some contrivances to an optical system portion such as a zoom lens and an electronic imaging system using the same, such as a video or digital camera. According to the present invention, the zoom lens is also designed to be capable of rear focusing.
In recent years, digital cameras (electronic cameras) have received attention as cameras of the next generation, an alternative to silver-halide 35 mm-film (usually called Leica format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type.
In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing video or digital cameras whose depth dimension is reduced while high image quality is ensured. The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially a zoom lens system from the surface located nearest to its object side to an image pickup plane. To make use of a collapsible lens mount that allows the optical system to be taken out of a camera body for phototaking and received therein for carrying now becomes mainstream.
However, the thickness of an optical system received in a collapsible lens mount varies largely with the lens type or filters used. Especially in the case of a so-called+precedent type zoom lens wherein a lens group having positive refracting power is positioned nearest to its object side, the thickness of each lens element and dead space are too large to set such requirements as zoom ratios and F-numbers at high values; in other words, the optical system does not become thin as expected, even upon received in the lens mount (JP-A 11-258507). A-precedent type zoom lens, especially of two or three-group construction is advantageous in this regard. However, this type zoom lens, too, does not become slim upon received in a collapsible lens mount, even when the lens positioned nearest to the object side is formed of a positive lens (JP-A 11-52246), because the lens groups are each composed of an increased number of lens elements, and the thickness of lens elements is large. Among zoom lenses known so far in the art, those set forth typically in JP-A""s 11-287953, 2000-267009 and 2000-275520 are suitable for use with electronic imaging systems with improved image-formation capabilities including zoom ratios, angles of view and F-numbers, and may possibly be reduced in thickness upon received in collapsible lens mounts.
To make the first lens group thin, it is preferable to make an entrance pupil position shallow; however, the magnification of the second lens group must be increased to this end. For this reason, some considerable load is applied on the second lens group. Thus, it is not only difficult to make the second lens group itself thin but it is also difficult to make correction for aberrations. In addition, the influence of production errors grows. Thickness and size reductions may be achieved by making the size of an image pickup device small. To ensure the same number of pixels, however, the pixel pitch must be diminished and insufficient sensitivity must be covered by the optical system. The same goes true for the influence of diffraction.
To obtain a camera body whose depth dimension is reduced, a rear focusing mode wherein the rear lens group is moved for focusing is effective in view of the layout of a driving system. It is then required to single out an optical system less susceptible to aberration fluctuations upon rear focusing.
In view of such problems as referred to above, the primary object of the invention is to thoroughly slim down a video or digital camera by singling out a zoom mode or zoom construction wherein a reduced number of lens elements are used to reduce the size of a zoom lens and simplify the layout thereof and stable yet high image-formation capabilities are kept over an infinite-to-nearby range, and optionally making lens elements thin thereby reducing the total thickness of each lens group and slimming down a zoom lens thoroughly by selection of filters.
According to the present invention, the aforesaid object is achievable by the provision of a zoom lens comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, wherein:
for zooming from a wide-angle end to a telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves with a varying spacing with the second lens group,
the second lens group comprises, in order from an object side thereof, a front subgroup and a rear subgroup with a space interposed therebetween, wherein the front subgroup consists of a doublet component consisting of, in order from an object side thereof, a positive lens element having an aspheric surface and a negative lens element, and the rear subgroup consists of one positive lens component, and
the zoom lens satisfies condition (1) with respect to the third lens group:
xe2x88x920.6 less than (R3F+R3R)/(R3Fxe2x88x92R3R) less than 1.2xe2x80x83xe2x80x83(1) 
where R3F is the axial radius of curvature of the surface of the third lens group located nearest to the object side thereof and R3R is the axial radius of curvature of the surface of the third lens group located nearest to the image side thereof.
The advantages of, and the requirements for, the aforesaid zoom lens arrangement are now explained.
The zoom lens of the present invention comprises, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power. For zooming from the wide-angle end to the telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves while the spacing between the second lens group and the third lens group varies. The second lens group comprises, in order from an object side thereof, a front subgroup and a rear subgroup with a space interposed therebetween, wherein the front subgroup consists of a doublet component consisting of, in order from an object side thereof, a positive lens element having an aspheric surface and a negative lens element, and the rear subgroup consists of one positive lens component.
In the present disclosure, the term xe2x80x9cdoublet or cemented lensxe2x80x9d should be understood to comprise a plurality of lens elements wherein a lens element formed of a single medium is thought of as one unit, and the xe2x80x9clens componentxe2x80x9d should be understood to refer to a lens group with no air separation therein, i.e., a single lens or a cemented lens.
For reductions in the size of a two-group zoom lens of xe2x88x92+ construction commonly used as the zoom lens for long-standing silver-halide film cameras, it is preferable to increase the magnification of the positive rear group (the second lens group) at each focal length. To this end, it is already well known to locate an additional positive lens component as the third lens group on the image side of the second lens group, wherein the spacing between the second lens group and the third lens group is varied for zooming from the wide-angle end to the telephoto end. The third lens group has also the possibility of being used for focusing.
To attain the object of the invention, i.e., to diminish the total thickness of a lens portion upon received in a collapsible mount yet perform focusing at the third lens group, it is important to reduce fluctuations of off-axis aberrations inclusive of astigmatism. To this end, the second lens group should preferably be composed of, in order from its object side, two lens components, i.e., a doublet component consisting a positive lens element having an aspheric surface (the front subgroup in the second lens group) and a positive lens component (the rear subgroup in the second lens group).
For focusing at the third lens group, aberration fluctuations become a problem. However, the incorporation of an aspheric surface in the third lens group in an amount than required is not preferable. This is because, to take advantage of that aspheric surface, astigmatism remaining at the first and second lens groups must be corrected at the third lens group. If, in this state, the third lens group moves for focusing, then aberrations are out of balance. Accordingly, when focusing is performed at the third lens group, astigmatism must be eradiated at the first and second lens group all over the zoom range.
It is thus preferable that the third lens group is constructed of a spherical lens component or a reduced amount of asphericity, an aperture stop is located on the object side of the second lens group, and the second lens group is composed of two lens components, that is, in order from its object side, a front subgroup made up of a doublet component consisting of a positive lens element having an aspheric surface and a negative lens element and a positive lens component (the rear subgroup).
Since the diameter of the front lens in this type can substantially be kept small, it is preferable to make the aperture stop integral with the second lens group (in the examples of the invention given later, the aperture stop is located just before the second lens group in a one-piece form), because not only is mechanical simplification achieved but also there is little or no dead space upon the lens portion received in a collapsible mount with a reduced F-number difference between the wide-angle end and the telephoto end. The positive lens element on the object side of the second lens group should preferably be cemented to the negative lens element because some considerable aberrations occur due to the relative decentration between them.
For focusing in particular, the following condition (1) should preferably be satisfied with respect to the third lens group that is movable.
xe2x88x920.6 less than (R3F+R3R)/(R3Fxe2x88x92R3R) less than 1.2xe2x80x83xe2x80x83(1)
Here R3F is the axial radius of curvature of the surface of the third lens group located nearest to the object side thereof and R3R is the axial radius of curvature of the surface of the third lens group located nearest to the image side thereof.
As the upper limit of 1.2 to condition (1) is exceeded, fluctuations of astigmatism with rear focusing become too large, and astigmatism with respect to a nearby object point is likely to become worse, although astigmatism at an infinite object point may be well corrected. As the lower limit of xe2x88x921.6 is not reached, correction of aberrations with respect to an infinite object point becomes difficult, although the fluctuations of astigmatism with rear focusing may be reduced.
Upon zooming from the wide-angle end to the telephoto end, the third lens group should preferably be designed to move in a convex locus toward the image side of the zoom lens, because it is easy to ensure a control margin at the telephoto end where focus position variations are large due especially to quality errors. The third lens group may also be designed to move in a convex locus toward the object side of the zoom lens.
The third lens group may also be composed of one positive lens component. Even so, practical-level correction of aberrations is feasible, making contributions to thickness reductions.
More preferably,
xe2x88x920.3 less than (R3F+R3R)/(R3Fxe2x88x92R3R) less than 0.9xe2x80x83xe2x80x83(1)xe2x80x2
Most preferably,
0 less than (R3F+R3R)/(R3Fxe2x88x92R3R) less than 0.6xe2x80x83xe2x80x83(1)xe2x80x3
Next, if the first lens group is composed of only two lens elements, i.e., a negative lens element having an aspheric surface and a positive lens element with satisfaction of the following conditions (2) and (3), it is then possible to make good correction for chromatic aberrations and Seidel off-axis aberrations, contributing to thickness reductions.
20 less than xcexd11xe2x88x92xcexd12xe2x80x83xe2x80x83(2) 
xe2x88x9210 less than (R13+R14)/(R13xe2x88x92R14) less than xe2x88x921.5xe2x80x83xe2x80x83(3)
Here xcexd11 is the d-line based Abbe number of the negative lens element in the first lens group, xcexd12 is the d-line based Abbe number of the positive lens element in the first lens group, and R13 and R14 are the axial radii of curvature of the object side- and image side-surfaces of the positive lens element in the first lens group, respectively.
Condition (2) defines fluctuations of longitudinal aberration and chromatic aberration of magnification during zooming. As the lower limit of 20 is not reached, the fluctuations of longitudinal aberration and chromatic aberration of magnification are prone to become noticeable. There is no particular upper limit because of the absence of any practically suitable medium; however, a prima facie upper limit to xcexd11xe2x88x92xcexd12 may be 75. A glass material exceeding the upper limit of 75 costs much.
Condition (3) defines the shape factor of the positive lens element in the first lens group. Falling short of the lower limit of xe2x88x9210 is not only unfavorable for correction of astigmatism but also requires an additional spacing between the first lens group and the second lens group so as to prevent mechanical interferences during zooming. Exceeding the upper limit of xe2x88x921.5 may possibly be unfavorable for correction of distortion.
More preferably, the following conditions (2)xe2x80x2 and/or (3)xe2x80x2 should be satisfied.
22 less than xcexd11xe2x88x92xcexd12xe2x80x83xe2x80x83(2)xe2x80x2
xe2x88x929 less than (R13+R14)/(R13xe2x88x92R14) less than xe2x88x922xe2x80x83xe2x80x83(3)xe2x80x2
Even more preferably, the following conditions (2)xe2x80x3 or (3)xe2x80x3 should be satisfied.
24 less than xcexd11xe2x88x92xcexd12xe2x80x83xe2x80x83(2)xe2x80x3
8 less than (R13+R14)/(R13xe2x88x92R14) less than xe2x88x922.5xe2x80x83xe2x80x83(3)xe2x80x3
Most preferably, both conditions (2)xe2x80x3 and (3)xe2x80x3 should be satisfied.
When the first lens group can be composed of only two lens elements as described above, substantial thickness reductions can be achieved by allowing the rear subgroup of the second lens group to be composed of a positive single lens component with satisfaction of the following condition (4).
0.7 less than t2/t1 less than 1.3xe2x80x83xe2x80x83(4) 
Here t1 is the axial thickness of the first lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof, and t2 is the axial thickness of the second lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof.
Increasing any spacing between the surfaces in each lens group is effective for correction of off-axis aberrations, especially astigmatism; however, this is not permissible for thickness reductions. The second lens group, on the other hand, is less susceptible to deterioration of off-axis aberrations due to the effect of the aspheric surface even when each spacing between the surfaces therein is reduced. In other words, the smaller the value of condition (4), the better the balance becomes. As the upper limit of 1.3 to that condition is exceeded, off-axis aberrations such as astigmatism cannot fully be corrected with a decreasing thickness of each lens group. As the lower limit of 0.7 is not reached, the second lens group cannot physically be set up, or the first lens group rather becomes thick.
More preferably,
0.8 less than t2/t1 less than 1.2xe2x80x83xe2x80x83(4)xe2x80x3
More preferably,
0.9 less than t2/t1 less than 1.1xe2x80x83xe2x80x83(4)xe2x80x3
In general, when the rear subgroup of the second lens group is composed of one positive single lens component with satisfaction of the following conditions (5), (6) and (7), it is possible to obtain a zoom lens that, albeit being slimmed down, can have satisfactory image-formation capability.
xe2x88x921.0 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.5xe2x80x83xe2x80x83(5) 
xe2x80x830.04 less than t2N/t2 less than 0.2xe2x80x83xe2x80x83(6)
xcexd22 less than 26.5xe2x80x83xe2x80x83(7) 
Here R2RF is the axial radius of curvature of the surface located nearest to the object side of the rear subgroup of the second lens group, R2RR is the axial radius of curvature of the surface located nearest to the image side of the rear subgroup of the second lens group, t2N is the axial thickness of the front subgroup of the second group to the object side-cementing surface thereof to the image side plane-surface thereof, t2 is the axial thickness of the second lens group from the surface nearest to the object side thereof to the surface located nearest to the image side thereof, and xcexd22 is the d-line based Abbe number of the negative lens element in the front subgroup of the second lens group.
Condition (5) defines the shape factor of the positive single lens component in the rear subgroup of the second lens group. As the lower limit of xe2x88x921.0 is not reached, correction of coma and astigmatism becomes difficult although the air separation d22 in the second lens group is easily made thin. As the upper limit of 0.5 is exceeded, mechanical interferences between the negative lens element in the front subgroup of the second lens group and the positive lens component in the rear subgroup of the second lens possibly cause that air separation d22 to become large, offering an obstacle to reducing the thickness of the zoom lens upon received on a collapsible lens mount.
Condition (6) defines the axial distance, t2N, from the image side-surface of the positive lens element located on the object side of the doublet component to the image side-surface of the negative lens element in the doublet component in the front subgroup of the second lens group. Unless this part has a certain thickness, astigmatism cannot fully be corrected; however, increasing the thickness of that part offers an obstacle to making each lens element thin. Accordingly, astigmatism should be corrected by the introduction of an aspheric surface to any surface in the first lens group. Nonetheless, falling short of the lower limit of 0.04 renders it impossible to make perfect correction of astigmatism. As the upper limit of 0.2 is exceeded, thickness increases unacceptably.
Condition (7) defines correction of longitudinal chromatic aberration and chromatic aberration of magnification. Exceeding the upper limit of 26.5 to condition (7) results in under-correction of longitudinal chromatic aberration. Although there is no particular lower limit to xcexd22 because of the absence of any practically suitable medium, a prima facie lower limit thereto may be 20. A glass material below that lower limit costs much.
More preferably, at least one or all of the following conditions (5)xe2x80x2, (6)xe2x80x2 and (7)xe2x80x2 should be satisfied.
xe2x88x920.9 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.2xe2x80x83xe2x80x83(5)xe2x80x2
0.06 less than t2N/t2 less than 0.18xe2x80x83xe2x80x83(6)xe2x80x2
xcexd22 less than 26xe2x80x83xe2x80x83(7)xe2x80x2
Even more preferably, at least one of the following conditions (5)xe2x80x3, (6)xe2x80x3 and (7)xe2x80x3 should be satisfied.
xe2x88x920.8 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0.05xe2x80x83xe2x80x83(5)xe2x80x2
0.08 less than t2N/t2 less than 0.16xe2x80x83xe2x80x83(6)xe2x80x3
xcexd22 less than 25.5xe2x80x83xe2x80x83(7)xe2x80x3
Most preferably, conditions (5)xe2x80x3, (6)xe2x80x3 and (7)xe2x80x3 should be all satisfied.
According to another arrangement for the second lens group, the rear subgroup may be composed of a doublet component consisting of, in order from its object side, a negative lens element and a positive lens element. In this embodiment, too, thickness reductions are achievable by satisfaction of condition (8) given below.
0.8 less than t2/t1 less than 1.5xe2x80x83xe2x80x83(8) 
Here t1 is the axial thickness of the first lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof, and t2 is the axial thickness of the second lens group from the surface located nearest to the object side thereof to the surface located nearest to the image side thereof.
The same requirement for condition (4) holds true for condition (8).
More preferably,
0.9 less than t2/t1 less than 1.4xe2x80x83xe2x80x83(8)xe2x80x2
Most preferably,
1.0 less than t2/t1 less than 1.3xe2x80x83xe2x80x83(8)xe2x80x2
When the rear subgroup of the second lens group is made up of the doublet component consisting of, in order from its object side, the negative lens element and the positive lens element, it is also preferable to satisfy conditions (9) and (10) given below.
xe2x88x921.5 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than 0xe2x80x83xe2x80x83(9) 
0 less than xcexd2RNxe2x88x92xcexd22 less than 35 wherein xcexd22xe2x89xa626.5xe2x80x83xe2x80x83(10) 
Here R2RF is the axial radius of curvature of the surface located nearest to the object side of the rear subgroup of the second lens group, R2RR is the axial radius of curvature of the surface located nearest to the image side of the rear subgroup of the second lens group, xcexd2RN is the d-line based Abbe number of the medium of the negative lens element in the rear subgroup of the second lens group, and xcexd22 is the d-line based Abbe number of the medium of the negative lens element in the front subgroup of the second lens group.
The same requirement for condition (5) holds true for condition (9).
Condition (10) is provided to make a well-balanced correction for longitudinal chromatic aberration and chromatic aberration of magnification. As the lower limit of 0 is not reached, the longitudinal chromatic aberration is susceptible to under-correction and the chromatic aberration of magnification to over-correction. As the upper limit of 35 is exceeded, the converse is true.
More preferably, the following conditions (9)xe2x80x2 and/or (10)xe2x80x2 should be satisfied.
xe2x88x921.4 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than xe2x88x920.1xe2x80x83xe2x80x83(9)xe2x80x2
5 less than xcexd2RNxe2x88x92xcexd22 less than 30 where xcexd22xe2x89xa626xe2x80x83xe2x80x83(10)xe2x80x2
Even more preferably, the following conditions (9)xe2x80x3 or (10)xe2x80x3 should be satisfied.
xe2x88x921.3 less than (R2RF+R2RR)/(R2RFxe2x88x92R2RR) less than xe2x88x920.2xe2x80x83xe2x80x83(9)xe2x80x3
10 less than xcexd2RNxe2x88x92xcexd22 less than 25 where xcexd22xe2x89xa625.5xe2x80x83xe2x80x83(10)xe2x80x3
Referring here to the aspheric surface introduced in the second lens group, the introduction of the aspheric surface to the front subgroup of the second lens group is effective for correction of spherical aberrations and coma. At the same time, this aspheric surface cooperates with a strong diverging surface in the front subgroup of the second lens group to cancel out aberration coefficients in that front subgroup, so that the relative decentration sensitivity of the front subgroup to the rear subgroup of the second lens group can be reduced. It is noted that correction of remaining coma, astigmatism, etc. by the introduction of an aspheric surface to the rear subgroup of the second lens group is not preferable because the relative decentration sensitivity of the rear subgroup to the front subgroup is rather increased. It is thus preferable that the rear subgroup of the second lens group consists only of spherical surfaces or an aspheric surface having a reduced amount of decentration is used thereat. In other words, the following condition (a) should preferably be satisfied.
0xe2x89xa6|Asp2R|xe2x89xa6fwxc3x9710xe2x88x923 (mm)xe2x80x83xe2x80x83(a) 
Here Asp21R is the amount of displacement of the aspheric surface from a spherical surface having a radius of curvature on the optical axis of all refracting surfaces in the rear subgroup of the second lens group, as measured at a position whose height from the optical axis is 0.8 time as small as the radius of the aperture stop at the telephoto end, and fw is the focal length of the zoom lens at the wide-angle end.
It is understood that the xe2x80x9camount of displacement of the aspheric surfacexe2x80x9d used herein refers to the amount of displacement of a certain aspheric surface with respect to a (reference) spherical surface having an axial radius of curvature, r, on that optical axis, as shown in FIG. 16, as measured at a position whose height from the optical axis is 0.8 time as small as the radius of the aperture stop at the telephoto end.
More preferably,
0xe2x89xa6|Asp2R|xe2x89xa60.5fwxc3x9710xe2x88x923 (mm)xe2x80x83xe2x80x83(a)xe2x80x2
Most preferably,
0xe2x89xa6|Asp2R|xe2x89xa60.2fwxc3x9710xe2x88x923 (mm)xe2x80x83xe2x80x83(a)xe2x80x3
The zoom lens of the present invention should preferably satisfies conditions (11), (12), (13) and (14) with respect to the second lens group in general.
0.6 less than R23/R21 less than 1.0xe2x80x83xe2x80x83(11) 
0.05 less than f2R/R22 less than 1.5xe2x80x83xe2x80x83(12) 
xe2x80x830.7 less than f2R/f2 less than 2xe2x80x83xe2x80x83(13)
0.01 less than n22xe2x88x92n21 less than 0.20xe2x80x83xe2x80x83(14) 
Here R21 is the axial radius of curvature of the surface located nearest to the object side of the front subgroup of the second lens group, R22 is the axial radius of curvature of the cementing surface in the front subgroup of the second lens group, R23 is the axial radius of curvature of the surface located nearest to the image side of the front subgroup of the second lens group, f2R is the focal length of the rear subgroup of the second lens group, f2 is the composite focal length of the second lens group, and n21 and n22 are the d-line based refractive indices of the media of the positive and negative lens elements in the front subgroup of the second lens group, respectively.
The doublet component that forms the front subgroup of the second lens group is provided to cancel out aberration coefficients therein to decrease the sensitivity to decentration. Exceeding the upper limit of 1.0 to condition 11 may be favorable for correction of spherical aberrations, coma and astigmatism throughout the zoom lens; however, the effect of cementing on slacking the sensitivity to decentration becomes slender. As the lower limit of 0.6 is not reached, the correction of spherical aberrations, coma and astigmatism throughout the zoom lens tends to become difficult.
Condition (12), too, provides a definition of correction of longitudinal chromatic aberration and chromatic aberration of magnification. As the upper limit of 1.5 to condition (12) is exceeded, it is easy to make the doublet component in the second lens group thin but correction of the longitudinal chromatic aberration becomes difficult. As the lower limit of 0.05 is not reached, favorable correction of the longitudinal chromatic aberration may be made; however, there is no option but to increase the thickness of the doublet component, offering an obstacle to reducing the thickness of the zoom lens portion upon received in a collapsible lens mount.
As the upper limit of 2 to condition (13) is exceeded, an exit pupil position comes close to the image plane, leading to the likelihood of shading and the relative decentration sensitivity between the front subgroup and the rear subgroup of the second lens group increases. As the lower limit of 0.7 is not reached, it is not only difficult to make sufficient correction of spherical aberrations, coma and astigmatism, but it is also difficult to ensure any high zoom ratio while maintaining compactness.
Condition (14) defines a difference in the index of refraction between the positive lens element and the negative lens element in the front subgroup of the second lens group. As the lower limit of 0.01 is not reached, general correction of coma and so on becomes difficult although the relative decentration sensitivity between the front subgroup and the rear subgroup in the second lens group may be decreased. Exceeding the upper limit of 0.20 may be favorable for correction of aberrations all over the zooming range; however, this is unfavorable for improving on the relative decentration sensitivity between the front subgroup and the rear subgroup in the second lens group.
More preferably, at least one or all of the following conditions (11)xe2x80x2, (12)xe2x80x2, (13)xe2x80x2 and (14)xe2x80x2 should be satisfied.
0.65 less than R23/R21 less than 0.95xe2x80x83xe2x80x83(11)xe2x80x2
0.2 less than f2R/R22 less than 1.4xe2x80x83xe2x80x83(12)xe2x80x2
0.75 less than f2R/f2 less than 1.9xe2x80x83xe2x80x83(13)xe2x80x2
0.02 less than n22xe2x88x92n21 less than 0.18xe2x80x83xe2x80x83(14)xe2x80x2
Even more preferably, at least one of the following conditions (11)xe2x80x3, (12)xe2x80x3, (13)xe2x80x3 and (14)xe2x80x3 should be satisfied.
0.7 less than R23/R21 less than 0.9xe2x80x83xe2x80x83(11)xe2x80x3
0.5 less than f2R/R22 less than 1.3xe2x80x83xe2x80x83(12)xe2x80x3
0.8 less than f2R/f2 less than 1.8xe2x80x83xe2x80x83(13)xe2x80x3
0.03 less than n22xe2x88x92n21 less than 0.16xe2x80x83xe2x80x83(14)xe2x80x3
Most preferably, these conditions (11)xe2x80x3, (12)xe2x80x3, (13)xe2x80x3 and (14)xe2x80x3 should be all satisfied.
The zoom lens of the present invention is favorable for setting up an electronic imaging system including a wide-angle area. In particular, the present zoom lens is preferable for use on an electronic imaging system wherein the diagonal half angle of view, xcfx89w, at the wide-angle end satisfies the following condition (this diagonal half angle of view is tantamount to the wide-angle-end half angle of view xcfx89w referred to in the examples given later):
27xc2x0 less than xcfx89w less than 42xc2x0
Being less than the lower limit of 27xc2x0 to this condition or the wide-angle-end half angle of view becoming narrow is advantageous for correction of aberrations; however, this wide-angle-end half angle is no longer practical. As the upper limit of 42xc2x0 is exceeded, on the other hand, distortion and chromatic aberration of magnification tend to occur and the number of lens elements increases.
With the present zoom lens used with the electronic imaging system of the present invention, off-axis chief rays are so almost vertically guided to the image pickup device that an image clear as far as its perimeter can be obtained. To reconcile an image of good quality with compactness, the diagonal length L of the effective image pickup area of the image pickup device should preferably be 3.0 mm to 12.0 mm inclusive.
Thus, the present invention provides means for improving the image-formation capability of the zoom lens part while diminishing the thickness the zoom lens part upon received in a collapsible lens mount.
Next, how and why the thickness of filters is reduced is now explained. In an electronic image pickup system, an infrared absorption filter having a certain thickness is usually inserted between an image pickup device and the object side of a zoom lens, so that the incidence of infrared light on the image pickup plane is prevented. Here consider the case where this filter is replaced by a coating devoid of thickness. In addition to the fact that the system becomes thin as a matter of course, there are spillover effects. When a near-infrared sharp cut coat having a transmittance (xcfx84600) of at least 80% at 600 nm and a transmittance (xcfx84700) of up to 8% at 700 nm is introduced between the image pickup device in the rear of the zoom lens system and the object side of the system, the transmittance at a near-infrared area of 700 nm or longer is relatively lower and the transmittance on the red side is relatively higher as compared with those of the absorption type, so that the tendency of bluish purple to turn into magentaxe2x80x94a defect of a CCD or other solid-state image pickup device having a complementary colors mosaic filterxe2x80x94is diminished by gain control and there can be obtained color reproduction comparable to that by a CCD or other solid-state image pickup device having a primary colors filter.
Thus, it is preferable to satisfy conditions (15) and (16):
xcfx84600/xcfx84550xe2x89xa70.8xe2x80x83xe2x80x83(15) 
xe2x80x83xcfx84700/xcfx84550xe2x89xa70.08xe2x80x83xe2x80x83(16)
Here xcfx84550 is the transmittance at a wavelength of 550 nm.
More preferably, the following conditions (15)xe2x80x2 and/or (16)xe2x80x2 should be satisfied:
xcfx84600/xcfx84550xe2x89xa70.85xe2x80x83xe2x80x83(15)xe2x80x2
xcfx84700/xcfx84550xe2x89xa70.05xe2x80x83xe2x80x83(16)xe2x80x2
Even more preferably, the following conditions (15)xe2x80x3 or (16)xe2x80x3 should be satisfied:
xcfx84600/xcfx84550xe2x89xa70.9xe2x80x83xe2x80x83(15)xe2x80x3
xcfx84700/xcfx84550xe2x89xa70.03xe2x80x83xe2x80x83(16)xe2x80x3
Most preferably, both conditions (15)xe2x80x3 and (16)xe2x80x3 should be satisfied.
Another defect of the CCD or other solid-state image pickup device is that the sensitivity to the wavelength of 550 nm in the near ultraviolet area is considerably higher than that of the human eye. This, too, makes noticeable chromatic blurring at the edges of an image due to chromatic aberrations in the near ultraviolet area. Such chromatic blurring is fatal to a compact optical system. Accordingly, if an absorber or reflector is inserted on the optical path, which is designed such that the ratio of the transmittance (xcfx84400) at 400 nm wavelength to that (xcfx84550) at 550 nm wavelength is less than 0.08 and the ratio of the transmittance (xcfx84440) at 440 nm wavelength to that (xcfx84550) at 550 nm wavelength is greater than 0.4, it is then possible to considerably reduce noises such as chromatic blurring while the wavelength area necessary for color reproduction (satisfactory color reproduction) is kept intact.
It is thus preferably to satisfy conditions (17) and (18):
xcfx84400/xcfx84550xe2x89xa60.08xe2x80x83xe2x80x83(17) 
xcfx84440/xcfx84550xe2x89xa70.4xe2x80x83xe2x80x83(18) 
More preferably, the following conditions (17)xe2x80x2 and/or (18)xe2x80x2 should be satisfied.
xcfx84400/xcfx84550xe2x89xa60.06xe2x80x83xe2x80x83(17)xe2x80x2
xcfx84440/xcfx84550xe2x89xa70.5xe2x80x83xe2x80x83(18)xe2x80x2
Even more preferably, the following condition (17)xe2x80x3 or (18)xe2x80x3 should be satisfied.
xcfx84440/xcfx84550xe2x89xa60.04xe2x80x83xe2x80x83(17)xe2x80x3
xcfx84440/xcfx84550xe2x89xa70.6xe2x80x83xe2x80x83(18)xe2x80x3
Most preferably, both condition (17)xe2x80x3 and (18)xe2x80x3 should be satisfied.
It is noted that these filters should preferably be located between the image-formation optical system and the image pickup device.
On the other hand, a complementary colors filter is higher in substantial sensitivity and more favorable in resolution than a primary colors filter-inserted CCD due to its high transmitted light energy, and provides a great merit when used in combination with a small-size CCD. Regarding an optical low-pass filter that is another filter, too, its total thickness tLPF (mm) should preferably satisfy condition (19):
0.15 less than tLPE/a less than 0.45xe2x80x83xe2x80x83(19) 
Here a is the horizontal pixel pitch (in xcexcm) of the image pickup device, and 5 xcexcm or lower.
Reducing the thickness of the optical low-pass filter, too, is effective for making the thickness of the zoom lens upon received in a collapsible mount; however, this is generally not preferred because the moirxc3xa9 preventive effect becomes slender. On the other hand, as the pixel pitch becomes small, the contrast of frequency components greater than Nyquist threshold decreases under the influence of diffraction of an image-formation lens system and, consequently, the decrease in the moirxc3xa9 preventive effect is more or less acceptable. For instance, it is known that when three different filters having crystallographic axes in directions where upon projected onto the image plane, the azimuth angle is horizontal (=0xc2x0) and xc2x145xc2x0 are used while they are put one upon another, some moirxc3xa9 preventive effect is obtainable. According to the specifications known to make the filter assembly thinnest, each filter is displaced by a xcexcm in the horizontal and by SQRT(xc2xd)*a xcexcm in the xc2x145xc2x0 directions. Here SQRT means a square root. The then filter thickness is approximately given by [1+2*SQRT(xc2xd)]*a/5.88 (mm). This is the specification where the contrast is reduced down to zero at a frequency corresponding just to Nyquist threshold. At a thickness a few % to a few tens of % smaller than this, a little more contrast of the frequency corresponding to Nyquist threshold appears; however, this can be suppressed under the influence of the aforesaid diffraction.
In other filter embodiments where two filters are placed one upon another or one single filter is used, too, it is preferable to meet condition (19). When the upper limit of 0.45 is exceeded, the optical low-pass filter becomes too thick, contrary to size reduction requirements. When the lower limit of 0.15 is not reached, moirxc3xa9 removal becomes insufficient. In this condition, a should be 5 xcexcm or less.
When a is 4 xcexcm or less or where the optical low-pass filter is more susceptible to diffraction, it is preferable that
0.13 less than tLPF/a less than 0.42xe2x80x83xe2x80x83(19)xe2x80x2
Depending on the number of low-pass filters put on the horizontal pixel pitch, it is also acceptable to meet condition (19)xe2x80x3:
0.3 less than tLPF/a less than 0.4xe2x80x83xe2x80x83(19)xe2x80x3
However,
0.2 less than tLPF/a less than 0.28 provided that three filters are placed one upon another and 4xe2x89xa6a less than 5 xcexcm,
0.1 less than tLPF/a less than 0.16 provided that two filters are placed one upon another and 4xe2x89xa6a less than 5 xcexcm,
0.25 less than tLPF/a less than 0.37 provided that three filters are placed one upon another and a less than 4 xcexcm,
0.16 less than tLPF/a less than 0.25 provided that two filters are placed one upon another and a less than 4 xcexcm, and
0.08 less than tLPF/a less than 0.14 provided that one filter is used and a less than 4 xcexcm.
When an image pickup device having a small pixel pitch is used, there is degradation in image quality under the influence of diffraction effect by stop-down. In this case, the electronic image pickup system is designed in such a way as to have a plurality of apertures each of fixed aperture size, one of which can be inserted into any one of optical paths between the lens surface located nearest to the image side of the first lens group and the lens surface located nearest to the object side of the third lens group and can be replaced with another as well, so that illuminance on the image plane can be adjusted. Then, media whose transmittances with respect to 550 nm are different but less than 80% are filled in some of the plurality of apertures for light quantity control. Alternatively, when control is carried out in such a way as to provide a light quantity corresponding to such an F-number as given by a (xcexcm)/F-number less than 4.0, it is preferable to fill the apertures with medium whose transmittance with respect to 550 nm are different but less than 80%. In the range of the full-aperture value to values deviating from the aforesaid condition as an example, any medium is not used or dummy media having a transmittance of at least 91% with respect to 550 nm are used. In the range of the aforesaid condition, it is preferable to control the quantity of light with an ND filter or the like, rather than to decrease the diameter of the aperture stop to such an extent that the influence of diffraction appears.
Alternatively, it is acceptable to uniformly reduce the diameters of a plurality of apertures inversely with the F-numbers, so that optical low-pass filters having different frequency characteristics can be inserted in place of ND filters. As degradation by diffraction becomes worse with stop-down, it is desirable that the smaller the aperture diameter, the higher the frequency characteristics the optical low-pass filters have.
A zoom lens such as one contemplated herein may be decreased in size with a diminishing size of an image pickup device used therewith.
To slim down a camera, it is effective to use the present zoom lens in combination with an electronic image pickup device that is small enough to satisfy condition (20):
Fxe2x89xa7axe2x80x83xe2x80x83(20) 
where a is the horizontal pixel pitch of the electronic image pickup device and F is a full-aperture F-number at the wide-angle end of the zoom lens. In this case, it is more preferable to rely on such contrivances as described below.
As the image pickup device becomes small, the pixel pitch becomes proportionally small, and so deterioration of image quality under the influence of diffraction is not negligible. Especially when the image pickup device is diminished to such a degree that the relation between the full-aperture F-number at the wide-angle end and the horizontal pixel pitch a (xcexcm) of the electronic image pickup device used meets the aforesaid condition (20), it is usable only in a full-aperture state. It is thus preferable that the aperture stop that determines the F-number has a fixed inner diameter and is kept against the insertion and de-insertion or replacement.
In addition, at least one of refracting surfaces adjacent to the aperture stop is located such that its convex surface (that is herein the refracting surface adjacent to the image side of the zoom lens) is directed toward the aperture stop and the point of intersection of the optical axis with a perpendicular from that aperture stop down to the optical axis is positioned within 0.5 mm from the apex of the convex surface or that convex surface intersects or contacts the inside diameter part of the aperture stop inclusive of the back surface of the aperture stop part. This contributes remarkably to size reductions because some considerable space so far needed for the aperture stop can be dispensed with and so considerable space savings are achievable.
As described above, it is preferable to use variable transmittance means instead of the aperture stop for the purpose of controlling the quantity of light. Because no particular problem arises in association with the location of the variable transmittance means on the optical path, it should preferably be inserted in any available space (e.g., between the second lens group and the third lens group or on the image plane side of the third lens group). For the present invention in particular, the variable transmittance means should preferably be inserted between the zooming lens group and the image pickup device.
For the variable transmittance means, it is acceptable to use means whose transmittance is variable depending on voltage or the like or a plurality of filters with varying transmittances, which are used in combination such that they can be inserted or de-inserted or replaced. Alternatively, a shutter for controlling the quantity of a light beam guided to the electronic image pickup device may be located in a space different from that for the aperture stop.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts that will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.